Analog-to-digital converters (ADCs) are used to convert an analog signal, more precisely a series of samples of an analog signal, to a series of digitally coded signals. ADCs span a spectrum of designs that range from parallel flash-type converters employing multiple comparators that generally require a single clock cycle to determine all bits of resolution at one end of the spectrum, to a successive approximation-type converter that generally requires one clock cycle per bit of resolution at the other end of the spectrum. In order to achieve a high speed rate of conversion with little latency, a large number of comparators are employed in a flash converter. While flash converters achieve a fast conversion, they consume relatively large amounts of area on integrated circuits and relatively large amounts of power.
Successive approximation converters consume relatively less area on integrated circuits and relatively less power than flash converters, but introduce latency in converting an analog sample to a digitally coded sample due to the sequential generation of bits. Successive approximation ADCs have been designed to operate on a fixed frequency clock, with the bit determination requiring the longest time being less than or equal to a clock cycle in duration. However, since not all bit determinations require the same amount of time, some time is wasted in a multibit successive approximation conversion. With the successive approximation ADCs clock cycle duration based on the worst-case bit determination time, all bit determinations, other than possibly the worst-case bit determination, wastes a portion of a clock cycle by completing the determination of a bit in less than the clock period allocated for that bit determination.
What is needed is a technique to improve the speed of conversion of a successive approximation ADC such as by providing a signal to drive the successive approximation bit determination based on the time required for each bit determination and not a predetermined duration of time allocated for each bit determination.